This invention relates to ink jet apparatus having a movable ink jet head for ejecting droplets of ink, and more particularly to such apparatus having a reservoir for supplying hot melt ink to the ink jet head.
The use in ink jet systems of hot melt ink, which ink is normally in a solid or frozen state but attains a liquid state or phase when its temperature is raised, has presented a number of advantages to ink jet apparatuses. For a discussion of the characteristics of such ink and the use thereof in ink jet apparatus, reference is made to U.S. Pat. No. 4,390,369 and pending U.S. application Ser. Nos. 093,151 filed Sept. 2, 1987, which is a continuation of Ser. No. 938,334, filed Dec. 4, 1986, and now abandoned, which in turn is a continuation of Ser. No. 610,627 filed May 16, 1984, now abandoned; Ser. No. 909,007, filed Sept. 16, 1986, which is a continuation of Ser. No. 803,038, filed Nov. 27, 1985, and now abandoned, which in turn is a continuation of Ser. No. 565,124, filed Dec. 23, 1983, now abandoned; and Ser. No. 644,542, filed Aug. 27, 1984, now U.S. Pat. No. 4,659,383 all assigned to the same assignee as this invention and incorporated herein by reference.
While the use of hot melt ink has presented advantages as discussed in the above references, it also creates additional requirements for the design of the apparatus, including with respect to the reservoir system. The reservoir which is part of the movable apparatus for devices such as ink jet printers, must be designed to maintain all of the ink in the reservoir at a substantially constant and uniform temperature so that the ink characteristics do not vary. Furthermore, there is a need to reduce fluid flow lengths, to protect against spillage of ink causing subsequent injury to the operator, and to maintain a substantially constant head of ink pressure regardless of movement of the reservoir.
One suitable such printer is described in copending U.S. application Ser. No. 829,572, filed Feb. 14, 1986, assigned to the assignee of the present invention and incorporated herein by reference. In that arrangement, an ink jet apparatus of the demand or impulse type comprises a chamber and an orifice from which droplets of ink are ejected in response to the state of energization of a transducer which communicates with the chamber through a foot forming a movable wall. The transducer expands and contracts, in a direction having at least one component extending parallel with the direction of droplet ejection through the orifice, and is elongated in such direction, the electric field resulting from the energizing voltage being applied transversed to the axis of elongation. Also provided for in that arrangement is a supply means of hot melt ink comprising a substantially cylindrically shaped pellet formed within a cartridge which is inserted within a receptacle for subsequent melting. In this manner, an operator is less likely to be burned by spilling ink since the ink is supplied to the printer in a cartridge form which is adapted to seal the ink supply, and thereby prevent spillage.
One of the major difficulties to the convenient use of hot melt inks in an ink jet device has been how to prevent the devices from depriming while cycling from their elevated operating temperatures to room temperature and back, through the melting/freezing temperature range of the ink. Numerous experiments have shown that even when a device is properly primed (i.e., filled and operating well with hot melt ink), cooling below the freezing temperature and warming it back to the operating temperature will often deprime it.
This phenomenon may be caused by several different effects. In one case, the ink may contract gradually as its temperature falls, resulting in a situation where the shrinking solid may either crack or fail to adhere to the ink passage walls thereby allowing air to penetrate the system. Furthermore, substantial shrinkage of the freezing ink may also pull air in through the ink jet orifices. Atmospheric gases, as well, can dissolve in the ink and cause bubbles to form on reheating since most gases are much less soluble in the solid ink than in the melted ink.
While the device of copending U.S. application Ser. No. 829,572 described herein above is suitable for use with hot melt ink, it nevertheless is susceptible to such depriming for the reasons discussed immediately above. It would, therefore, be desirable to provide an improved reservoir system which could be integrally mounted with the ink jet head, and which would provide optimum flow of the hot melt ink to the ink jet head while minimizing the effects of depriming caused by the freezing of the ink.